EARLY ON THE CALM AND misty morning of Sept. 6, 1940, with the Battle of Britain raging and a German invasion of England imminent, the Canadian liner Duchess of Richmond slipped into Halifax Harbor in Nova Scotia. Aboard was a small party of British officers and scientists assigned to escort a black metal box that held valuable, but raw, military technologies. Chief among these treasures was Britain's most precious secret: a revolutionary radar transmitter called the cavity magnetron.

The magnetron went straight to Washington, kicking off one of the century's seminal technological thrillers. Some 30 months before the atomic-bomb project opened in Los Alamos, N.M., top U.S. physicists were secretly recruited to develop radar devices based on the British invention. Their pioneering systems--for bombing through overcast, hunting U-boats, gun control and much else--proved so critical to the Allied cause that the American historian James Phinney Baxter III proclaimed the magnetron "the most valuable cargo ever brought to our shores." And Baxter made that assessment in 1946, well before radar's peacetime spinoffs became apparent. In addition to prompting enormous advances in air-traffic control and weather forecasting, radar contributed directly to Nobel Prizes for nuclear magnetic resonance (the basis for the MRI machines in today's hospitals), the transistor and the maser, an extremely powerful amplifier that led to the laser. It vastly expanded our knowledge of the cosmos, helping reveal the strange universe of pulsars and quasars. Radar even gave busy cooks the microwave oven. Says Dale R. Corson, president emeritus of Cornell University, who cut his teeth on the magnetron project: "Scientists-turned-engineers learned to manipulate microwaves in a thousand ingenious ways and won World War II. After the war they used the technology to open scientific worlds and create whole new industries. It is a fabulous legacy.

"Radar" is an acronym for radio detection and ranging. In a nutshell, the technology involves sending out a radio-wave pulse. If the waves strike a ship, plane or other object, a portion of the energy is reflected back to the transmitting station. Since radio signals travel at the speed of light, timing the journey allows the object's distance to be calculated. Various other tricks, such as measuring the angle and direction of incoming echoes, can pinpoint a target's exact location.

Though the magnetron marked a critical advance, a primitive form of radar, powered by more conventional transmitters, was invented long before World War II. In 1904 German engineer Christian Hillsmeyer patented the short-lived telomobiloskop, a collision-prevention device for ships that worked much like a searchlight only with radio waves in place of visible rays. The first modern system was cobbled together in December 1934 at the Naval Research Laboratory in Washington, but others were soon under development in Germany, France, Russia, Italy and Britain.

Only the British made radar the backbone of their defense system, in January 1935, alarmed by Hitler's rise to power, government advisers asked atmospheric researcher Robert Watson-Watt to assess the feasibility of a death ray that might zap enemy pilots in the cockpit. The Scotsman dismissed that notion but suggested instead what he called "radio detection as opposed to radio destruction." A secret project was launched, and by the time war was declared, Britain had deployed its Chain Home network, radar towers up to 800 feet tall that ringed the country's south and east coasts, able to spot enemy bombers about 100 miles off.

Even as the black box arrived in America, the network provided the crucial edge in the Battle of Britain. But the Chain used radio signals with a long wavelength of about 10 meters. This meant that the system worked best in daylight, when a pilot's sharp eyes could correct for errors of several miles inherent in long wavelengths. The magnetron was designed to correct this weakness as the Germans moved to night attacks. Its wavelength was 100 times shorter--10 centimeters--and it could be fitted onto fighter planes so pilots could find their quarries on even the darkest nights.

But war-strapped British industry was unable to perfect the still roughshod magnetrons, so Prime Minister Winston Churchill reached out to his future American allies. The United States hurriedly established a secret lab at the Massachusetts Institute of Technology to exploit the magnetron. Dubbed the Radiation Laboratory, the facility opened in November 1940, staffed by a small body of the country's top physicists, among them: director Lee DuBridge, a future Caltech president; Columbia University's I. I. Rabi, and Luis Alvarez, later famous for his theory that asteroids caused the dinosaurs' extinction. In all, 10 of these recruits would win the Nobel Prize.

They worked fast and well, bolstered by innovations from military and industrial organizations. In 1942 conventional radar--without magnetrons--provided crucial warning of Japanese air attacks at the Battles of Midway and the Coral Sea; the Imperial Navy had no such edge. Then, in 1943, planes equipped with the Rad Lab's magnetron systems began patrolling the North Atlantic and turned the tide against Germany's U-boats, which had devastated Allied shipping in the early years of the war. Equipped with radar, American and British planes were able to track and destroy the submarines. American submarines hunting Japanese merchantmen with microwave radar did more damage than the U-boats. Radar-targeted bombing took center stage in Europe and the Pacific. The Rad Lab's airborne interception radar remained the Royal Air Force standard until 1957. Its navigation system became the worldwide grid Loran, which is still active. By the end of the war, many weapons experts concluded that the atomic bomb had only ended the war, radar won it.

And the war supplied only half the story of radar's triumphs. After the Japanese surrender in August 1945, the Rad Lab published a 28-volume series recording all the advances spawned by the new technology in such areas as antennas, wave propagation, waveguides and transmitters. The spinoffs came quickly. AT&T relied on radar know-how to extend its microwave long-distance telephone network. Late in 1945 Raytheon engineer Percy Spencer reportedly stood near a working magnetron and noticed his candy bar melting. That led to the Radarange, the first microwave oven. Even today, magnetrons form the core of these popular kitchen appliances.

Luis Alvarez immediately began plotting a linear accelerator powered by 600 surplus radar oscillators. His Microwave Early Warning radar became the main air-traffic-control system for guiding planes between airports, while his Ground Controlled Approach radar landing set proved vital in guiding planes through heavy fogs during the 1949 Berlin airlift.

A single wartime event provided the key to several postwar discoveries. In the winter of 1943-44 the Rad Lab's top priority was producing a bombing radar at the extremely short wavelength of 1.25 cm. Everything went beautifully, but by April the echoes had grown noticeably weaker. It turned out that in the increasing humidity of spring, water-vapor molecules in the atmosphere were absorbing part of the signal.

That discovery inspired Edward Purcell's pioneering experiment with nuclear magnetic resonance on Dec. 10, 1945: Purcell, who had led the Rad Lab investigation into the vanishing-signal mystery, realized that by studying the energy absorbed by protons instead of molecules, one could unveil the inner workings of the hydrogen atom. This paved the way for several advances, including magnetic resonance imaging machines, which measure the energy absorbed by different tissues--including tumors--to produce detailed internal images of the body. Through a more indirect route, the same phenomenon set Bell Labs radar researcher Charles Townes on the path to the maser.

Radar quickly made its mark on a variety of other fronts. Wartime studies aimed at understanding and improving the sensitive semiconductor crystals used to detect radar echoes laid critical groundwork for the invention of the transistor in December 1947. The technology was also turned skyward, as electronics veterans scavenged surplus equipment to obtain images of meteor showers and other astronomical phenomena. By the early 1960s, radar signals had been bounced off Venus to provide the first highly accurate measurement of the Earth-Venus distance, from which came a greatly improved solar-system model. Radar's subsequent triumphs include mapping the Venusian surface, most dramatically via the spacecraft Magellan.

A variation on radar astronomy is radio astronomy. Instead of bouncing signals off targets, scientists listen for the radio "noise" streaming in from pulsars, quasars and a variety of unseen objects. Soon after World War II, Australian researchers led by Edward (Taffy) Bowen, who had helped escort the cavity magnetron to America, used wartime radar equipment to detect the first "radio stars," which turned out to be supernovas and colliding galaxies.

Back on Earth, radar began snaring speeders. To the delight of the press, a Canadian policeman caught Robert Watson-Watt with a radar device :in 1954, He didn't contest the ticket. Just paid his fine and went on his way.


Radar detects a distant object by emitting radio waves much as bats emit sound waves. Echoes produced when the waves bounce off an object provide information about its exact location.

How radar was invented

Scottish scientist Robert Watson-Watt revolutionized warfare with his conception of the first radar defense system in 1935. By 1939, Britain had installed an extensive warning network on its shores to detect enemy planes before they could attack. Scientists had been tinkering with radio waves long before; in 1887, Heinrich Hertz generated and detected the first such waves in a lab on his homemade oscillator. (Frequency units still bear his name today.) Hertz's work led to radio communications in the late 1890s and provided the foundation for the invention of radar.

Anatomy of a radar wave

Frequency is determined by wavelength, or the distance covered in one crest-to-crest cycle. Frequency is expressed in cycles per second, or Hertz (Hz).

In World War II, radar helped night fighters defend Britain

Early fighter planes sent out radar signals from the nose and received echoes on each wing. If returned signals were unbalanced, the target was off to one side or above or below the fighter plane.

When incoming signals were equal, the target was dead ahead.

From making instant popcorn to finding distant galaxies, radar waves play a role from the home to the heavens

Air-traffic control Signals help controllers monitor and land planes.

Weather predictions Doppler radar can tell hurricanes from tornadoes.

Microwave cooking Microwaves radiate through food to heat it quickly.

Speed of baseball Radar guns measure pitches, also highway speeders.

Subterranean search Waves travel up to 100 ft. below ground.

Burglar alarms Sensors detect the movements of an intruder.

Radiotelescopes They spot pulsars, quasars and other distant objects.